The high-ceilinged room resembles a brightly lit, white-walled operating theatre, and the people inside are dressed for surgery. It is impossible to tell if they are men or women – or even human – they are so enveloped in white smocks, masks, boots and gloves; their heads covered in hoods, even their eyes obscured by glasses.

They work as precisely as if they were performing an operation. But they are not operating on a patient, they are operating on a spacecraft. A cuboid mass of shiny silver-coloured metal, 2-metres (6.5-feet) high and covered in delicate instruments and antennae. As I watch, these engineers busy themselves attaching this unique machine to its life-supporting solar arrays.

I am witnessing the final stage of assembly of Maven at the Lockheed Martin Space Systems facility in Denver, Colorado. Shortly to be shipped off for launch, due later this year, this Nasa mission is designed to answer fundamental questions about the Martian atmosphere. And the outfits worn by the engineers in this spacecraft factory will ensure that no cells, bacteria or chemicals from Earth contaminate the destination planet.

Maven (pronounced May-ven) is a somewhat tortured acronym for Mars Atmosphere and Volatile EvolutioN. Evidence from the Curiosity rover, along with other recent missions, has led scientists to deduce that Mars once had the conditions suitable for life, including liquid water. As Curiosity continues to trundle its way slowly across Gale Crater, Maven will orbit the planet to examine whether the atmosphere could also have provided life support.

“What we know from our missions looking at the surface of Mars is that there used to be water there,” says Guy Beutelschies, Maven Programme Manager at Lockheed Martin. “We can see the outlines of ancient rivers, the shorelines of ancient oceans. But water can’t exist there now – the atmosphere is too thin and too dry, any water would just evaporate or freeze.” So the big question is what happened to Mars’ atmosphere? “It used to be thicker, warmer, wetter, now it’s thin and dry,” says Beutelschies, before adding: “how did we get there?”

A letter published last week in the journal Nature adds weight to the argument that Mars once had an oxygen-rich atmosphere, though other scientists remain sceptical. Today, that atmosphere is equivalent to just one-hundredth the thickness of Earth’s. Made up mostly of carbon dioxide, with a tiny fraction of water vapour, this fragile, thin band around the red planet is now unlikely to support any sort of life, as far as we know. But the atmosphere in the past must have been more substantial to allow the formation of rivers, lakes and oceans. Scientists working on the Maven mission want to understand what this atmosphere was like, in as much detail as possible, and the processes that led to its destruction.

Short of being able to travel back in time into the Martian past, how do you go about tackling these questions with a mission today?

“In a sense we are building a little bit of a time machine,” says Beutelschies. “What we’re doing is understanding the processes.” And with that information, they can work backwards to track how the atmosphere has changed and build a computer model, or simulation, of what it used to be like.

Top-down view

Today, what little remains of Mars’ atmosphere is being stripped away by the solar wind – the stream of charged particles continuously spewed out by the Sun. The Earth is protected from the solar wind by a magnetic bubble surrounding our planet, known as the magnetosphere. This is generated by the Earth’s magnetic field, which effectively turns our planet into a giant bar magnet. Mars does not have a field or, at least, it doesn’t have one anymore.

“We think that Mars used to have a magnetic field,” explains Bruce Jakosky, the Principal Investigator for Maven, who is based at the University of Colorado’s Laboratory for Atmospheric and Space Physics in Boulder. “We see places on the surface that retain some remnant magnetism, they were imprinted when they formed with whatever magnetism was there. We think that some four billion years ago, when the magnetic field turned off, that turn-off of the magnetic field allowed turn-on of the stripping by the solar wind.”

To investigate the processes taking place today, Maven will dip into the tenuous Martian upper atmosphere with each orbit, measuring the particles, sampling gases, monitoring the magnetic field and solar wind. “Rover missions for the past fifteen years have focused on habitability and the history of habitability,” says Jakosky. “We’re trying to get the other half of the problem, what role the top of the atmosphere has played. The rovers get the bottom of the atmosphere, we get the top of the atmosphere and how it interacts with the Sun.” Both are needed to put together a picture of what’s controlling the Mars environment. Results from the Curiosity rover published in Science last week, for instance, supports the evidence that Mars may have lost most of its atmosphere billions of years ago.

As well as filling in the blanks about Mars' depleted atmosphere, Maven will also provide clues to the habitability of other planets, beyond the solar system. “In trying to understand the distribution of life throughout the Universe, this is a really important indicator,” says Jakosky. “Understanding the environmental conditions that allow [life] to exist, or don’t allow it to exist, is key to being able to extrapolate elsewhere.”

Right now, Mars is being studied in unprecedented detail – from spacecraft in orbit and by rovers on the ground. Maven will provide another part of the jigsaw as we build up an understanding of a planet that, it is now becoming clear, used to be remarkably like our own.

“Imagine that!” exclaims Beutelschies. “We had a solar system with two planets with oceans on them! That certainly raises the question in many people’s minds about whether Mars could have had life sometime in the past.”

For Mars it, somehow, all went horribly wrong. We should just count ourselves lucky we evolved on Earth instead.

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